The tumor-promoting transcription factor hypoxia-inducible factor-1 (HIF-1) alpha creates conditions favoring tumor growth and metastases. HIF is a highly attractive candidate for pharmacological intervention because of its overexpression in a majority of primary and metastatic tumors. HIF overexpression is also found concomitantly with mutational inactivation of VHL, which is the causative agent for the majority of renal cell carcinomas (RCC), the most common manifestation of kidney cancer. One promising therapeutic approach is use of Hsp90 antagonists, which we have shown to be effective inhibitors of HIF activity. HIF interacts with the molecular chaperone heat shock protein 90 (Hsp90) and Hsp90 antagonists such as geldanamycin (GA) reduce hypoxia-induced HIF activity by promoting chaperone-mediated destruction of the protein. The Hsp90-dependent regulation of HIF activity takes center stage in the absence of VHL-mediated degradative pathways, however, knowledge of the role Hsp90 plays in HIF regulation is lacking, which presents a potential limitation to optimization of clinical use of Hsp90 antagonists towards HIF. In support of this notion, preliminary data is presented herein demonstrating that interaction of HIF with transcriptional coactivators attenuates the GA sensitivity of HIF. This model predicts that activation of HIF coincides with a dissolution of HIF-Hsp90 complexes, with subsequent modulation of GA sensitivity and HIF turnover. Furthermore, phosphorylation of HIF correlates with the activated, presumably Hsp90-free pool of protein, suggesting that HIF phosphorylation may act as part of the molecular switch regulating chaperone association. A variety of techniques, including deletion analysis and mutagenesis, will be used to identify the domains required for HIF's sensitivity to GA, the specific sites subject to phosphorylation, and how these alterations modulate Hsp90 association. This proposal will elucidate the role of Hsp90 in HIF function and explore signaling pathways that regulate HIF association with Hsp90. Given that Hsp90 modulates unknown functions of HIF, and Hsp90 antagonists are undergoing clinical trials, an understanding of how Hsp90-targeted drugs impact upon essential HIF-dependent functions is a clinically important question. The proposed experimental strategy will contribute to the long-range goal of maximizing the therapeutic application of Hsp90 antagonists towards HIF. [unreadable] [unreadable]